Field of the Invention
The present invention relates to the field of aerogel synthetic chemistry. More particularly, the present invention in embodiments relates to a one-step method for producing an aerogel which does not require solvent exchange.
Description of Related Art
Aerogels are solid materials of extremely low density, produced by removing the liquid component from a conventional wet gel. They are ultra-light, highly porous and highly thermally insulating materials composed of a network of interconnected nanostructures. Their typical density is lower than 0.1 g/cm3, their surface area is in the 700-1000 m2/g range and their thermal conductivity can be as low as 2.1 mW/mK (see N. Leventis, Accounts of Chemical Research., 2007, 40, 874 (“Leventis, 2007”) and A. C. Pierre, G. M. Pajonk, Chem. Rev., 2002, 102, 4243 (“Pierre, 2002”)). Because of this unique combination of properties, aerogels are being considered for applications as varied as thermal and sound insulation for the aerospace industry, as absorbents for environmental remediation and as catalyst supports. However, aerogels are also mechanically fragile and their use has been limited to niche applications such as thermal insulation for the Mars Rovers, as collectors of space and comet dust and as Cerenkov detectors (see Leventis, 2007 and Pierre, 2002). By way of background, other efforts in this area include those described by Leventis et al., such as in U.S. Pat. No. 8,227,363 and in U.S. Patent Application No. 2011/0250428 A1, hereby incorporated by reference in their entireties.
Aerogels are fabricated starting from wet gels. Wet gels are porous materials with the same porosity and surface area of aerogels. However, the pores of wet gels are filled with solvent and precursors used for the synthesis. Typically, the solvent is some alcohol (methanol, ethanol, propanol) and some water is added to catalyze the synthetic reaction. The solvent cannot be evaporated without cracking the gel because of capillary forces. That is, the solvent adheres strongly to the pore walls and induces cracks and pore collapse when it evaporates. To prevent cracking, a fluid with a low (ideally zero) surface tension is employed, which minimizes the capillary forces. This solvent is typically a supercritical solvent. For example, a method described in U.S. Pat. No. 7,384,988 to Anderson (incorporated by reference herein in its entirety) provides for preparation of aerogels using rapid supercritical extraction (RSCE) using a mold to contain gelation solution under a desired pressure and temperature in order to form the aerogel, then excess solvent (supercritical alcohol) escapes through gaps in the mold or through a relief valve. For a series of technical reasons (low supercritical pressure and temperature, low cost, low toxicity, low flammability), supercritical CO2 is the most popular choice. For supercritical drying, a wet gel is placed into liquid CO2, which replaces the solvent when it diffuses inside the pores. The vessel containing the liquid CO2 is then heated, the liquid CO2 becomes supercritical and crack-free aerogel monoliths are produced.
Drying in supercritical CO2, however, presents several disadvantages. In the first place, water is not soluble in liquid CO2, thus, CO2 will not diffuse inside the pores if water is present. This requires exchange of the solvents used in the gel synthesis with a water free solvent, the most popular choice being acetone. For example, Douglas A. Loy et al., Direct Formation of Aerogels by Sol-Gel Polymerizations of Alkoxysilanes in Supercritical Carbon Dioxide, Chem. Mater. 1997, 9, 2264-2268 (incorporated by reference herein in its entirety), addresses this by eliminating the organic (alcohol) solvent altogether. Furthermore, solvent exchange is an extremely time-consuming process. Depending on the size of the gel, it may take several days, and require a volume of fresh solvent 5-10 times larger than the volume of the gel. In addition, the time required for solvent exchange scales roughly with the square of the dimensions of the gel. For gels with dimensions larger than about 1 inch, the time required for exchange can be of almost one week. Because of this, industrial manufacturers of aerogels produce parts with one linear dimension of a few millimeters to limit the exchange times to a few hours. When considering diffusion times, one must also consider the time required for diffusion of liquid CO2. For large parts, this time can also be of days, and forces one to keep the drying vessel refrigerated and at high pressures (˜800 psi) for days in a row, quite an impractical proposition. Given these limitations, there is a need in the art for improved processes for producing aerogels.